1. Field of the Invention
The present invention relates to a method for manufacturing a photocatalytic titanium dioxide powder, and more specifically to a method for manufacturing a photocatalytic titanium dioxide powder for use in wastewater treatment and air purification.
2. Description of the Prior Art
Generally, wastewater which is discharged by industries is comprised of organic substances commonly constituted with carbon, hydrogen, nitrogen, oxygen, and sulfur. To treat wastewater having such organic substances which are difficult to decompose, UV-ray and photocatalysts of titanium dioxide, zinc oxide, iron oxide, cadmium sulfate, or zinc sulfate are conventionally used to decompose organic substances which do not readily decompose. Amongst photocatalysts, titanium dioxide is most appropriate and widely used to treat wastewater.
Titanium dioxide also has a wide applications in pigments, inorganic membranes, and semiconductor industries. Titanium dioxide is commonly found in both anatase and rutile phase crystalline structures. In its rutile phase, it is used a white pigment and has an excellent dispersion effect of intercepting UV-rays. Further, because of its high dielectric constant in the rutile phase, titanium dioxide is used as a dielectric material in semiconductors, while it is used as a coating material for optical lenses due to its high refractive index. It also showed chemical stability in the presence of strong acids and bases.
In its anatase crystalline phase, titanium dioxide exhibits a high photo-activity and is commonly used in a system for optically decomposing trichloro-ethane and in a system for transferring solar energy as a photocatalyst.
Currently, titanium dioxide is commercially manufactured by utilizing a sulfate process or a chloride process. In sulfate process, ilmenite which is utilized as a starting raw mineral is pulverized and the resulting ground material is dissolved with sulfate to obtain anatase phase titanium dioxide. In chloride process, a natural or a synthesized raw material having 90% pure rutile phase is chlorinated to obtain titanium tetrachloride (TiCl.sub.4), which is then reacted with oxygen to yield titanium dioxide having a rutile phase crystalline structure. Such chloride process for manufacturing titanium dioxide is disclosed by U.S. Pat. No. 5,698,177. Because the degree of the photocatalytic ability of the titanium dioxide power is mainly determined by its surface area, titanium dioxide powder particles having a smaller size and a greater specific surface area exhibit a higher photocatalytic ability.
In utilization of sulfate process to manufacture titanium dioxide, the impurities introduced during the manufacturing process tend to deteriorate the quality of the powder, while chlorine process which utilizes an erosive gas during the process necessitates having complex accompanying equipments, thus increasing the manufacturing cost for the titanium dioxide powder. Further, the powders obtained by the above processes have an average particle diameter of 200-400 nm and a specific surface area of 8-10 m.sup.2 /g, which are somewhat lacking and desire to be improved for optimizing the photocatalytic ability of titanium dioxide powder.
To be effectively used a photocatalyst, it is highly desired that the titanium dioxide powder has a specific surface area of greater than 50 m.sup.2 /g, an average diameter of the second particle of 100 nm, and a highly pure anatase crystalline phase. To manufacture such titanium dioxide power, a conventional method which utilizes Ti(OC.sub.3 H7).sub.4, Ti(OC.sub.2 H.sub.5).sub.4 or titanium alkoxides as a starting material has been disclosed. In the method, the starting material is subjected to hydrolysis and condensation processes to obtain the resulting titanium dioxide. Such method has significant drawbacks in that the starting material is expensive and the process requires a strict manufacturing control to obtain the desired product.
Further, the chlorine process described earlier present a problem of having necessary equipments to deal with safety and toxicity of the starting materials, while its resulting product is significantly constituted with rutile phase titanium dioxide powder, making it unsuitable to be utilized as photocatalyst.
Recently, there has been proposed a method of utilizing titanium dioxide or meta-titanic acid with an additive to obtain titanium dioxide power for use in wastewater treatment. In this method, the above starting material mixed with the additive is subjected to a ball-mill process, then sintered to obtain the desired titanium dioxide powder. For example, Korea Patent Laid-Open Publication No. 98-684 discloses a method of utilizing the above titanium dioxide or meta-titanic acid with at least one additive from carbonates and hydroxides. In the method, the starting material having the additive is ball-milled and then sintered at 550-1100.degree. C. to obtain the final titanium dioxide powder. Still, the titanium dioxide powder obtained the above process is inadequate to be used as a photocatalyst due to the small specific surface area and large average size of its particles.